XP can be remote managed from w2k3 but not from XP

I have a few Windows XP machines on the LAN that I am unable to access
remotely in computer management from other Windows XP machines.
However, from Windows 2003 Server I can access them correctly. I'm
logged in as domain admin both when connecting from w2k3 and when
trying from XP. I'm also failing to connect to their registries from
other XP machines but not from w2k3. And same problem using psexec,
trying a net use to connect to a share, etc.

Plenty of other XP machines on the LAN can be accessed fine. The only
common point I can find between the machines which I can't reach is
that they at some stage all had ZoneAlarm installed even it has since
been removed. ZA was never installed on any of the machines to which I
can connect successfully. No other firewall is running on them (XP
firewall is disabled through GPO) and anyway they're not blocking
everything cos I can get to them from w2k3.

Any ideas of where to look to resolve this?

Thanks in advance
Chris